Sodium secondary battery and electrical device

Abstract

A sodium secondary battery, the sodium secondary battery comprising a positive electrode sheet, an electrolyte and a negative electrode sheet; the positive electrode sheet comprises a positive electrode current collector and a positive electrode active material layer provided on at least one side of the positive electrode current collector, the positive electrode active material layer comprising a positive electrode active material and a first fast ion conductor, and the first fast ion conductor comprising one or more of sodium pyrophosphate, sodium manganese vanadium phosphate, and sodium titanium manganese phosphate; the electrolyte comprises a solvent, and the solvent comprises a linear ether and a cyclic ether.

Description

Sodium secondary batteries and electrical equipment

[0001] Priority information

[0002] This application claims priority and benefit to patent application 202411896860.8, filed with the China National Intellectual Property Administration on December 20, 2024, the entire contents of which are incorporated herein by reference. Technical Field

[0003] This application belongs to the field of secondary batteries, specifically relating to a sodium secondary battery and an electrical device. Background Technology

[0004] Secondary batteries are widely used in energy storage power systems such as hydropower, thermal power, wind power and solar power plants, as well as in many fields such as power tools, electric bicycles, electric motorcycles, electric cars, military equipment, and aerospace.

[0005] Sodium-ion batteries, as a promising energy storage technology, have shown broad development prospects due to their abundant raw materials, low manufacturing costs, and strong temperature adaptability. However, due to their system characteristics, dendrites easily form on the negative electrode in sodium-ion batteries. The cumulative growth of these dendrites can cause short circuits (internal short circuits) inside the battery, leading to rapid capacity degradation and severely limiting the cycle stability of sodium-ion batteries. Summary of the Invention

[0006] In view of the technical problems existing in the background art, this application provides a secondary battery that aims to suppress the internal shortness caused by the growth of sodium dendrites and improve the cycle performance of sodium secondary batteries.

[0007] To achieve the above objectives, the first aspect of this application proposes a sodium secondary battery, the sodium secondary battery comprising a positive electrode, an electrolyte, and a negative electrode, wherein the positive electrode comprises a positive current collector and a positive active material layer disposed on at least one side of the positive current collector, the positive active material layer comprising a positive active material and a first fast ion conductor, the first fast ion conductor comprising one or more of sodium pyrophosphate, sodium manganese vanadium phosphate, and sodium titanium manganese phosphate; the electrolyte comprises a solvent, the solvent comprising linear ethers and cyclic ethers.

[0008] This application includes at least the following beneficial effects: In the sodium secondary battery of this application, the use of a first fast ion conductor in the positive electrode active material layer, combined with linear ethers and cyclic ethers of the electrolyte solvent, can reduce the internal short circuit caused by the growth of sodium dendrites in the sodium secondary battery and improve the cycle performance of the sodium secondary battery.

[0009] In some embodiments, the sodium secondary battery satisfies one or more of the following conditions: the sodium pyrophosphate salt includes Na. 4+aM 3+b (PO4) 2+c (P2O7) 1+d Wherein, M includes at least one of Fe, Co, Mn or Ni, and -0.1≤a≤0.1, -0.1≤b≤0.1, -0.1≤c≤0.1, -0.1≤d≤0.1; the sodium manganese vanadium phosphate includes Na 3+f Mn x V 2-x (PO4) 3+y1 Wherein, -0.1≤f≤0.1, 0≤x≤1, -0.1≤y1≤0.1; the sodium titanium manganese phosphate includes Na 3+x1 MnTi 1-x1 V x1 (PO4) 3+e Where 0≤x1≤1, -0.1≤e≤0.1. Therefore, the internal short circuit caused by sodium dendrite growth in sodium secondary batteries can be reduced, thus improving the cycle performance of sodium secondary batteries.

[0010] In some embodiments, the positive electrode further includes a second layer disposed on the side of the positive electrode active material layer opposite to the positive electrode current collector. This second layer includes a second fast ion conductor, which may be one or more of sodium pyrophosphate, sodium manganese vanadium phosphate, and sodium titanium manganese phosphate. This reduces the internal short circuit caused by sodium dendrite growth in the sodium secondary battery, thereby improving its cycle performance.

[0011] In some embodiments, the residual alkali in the first fast ion conductor accounts for 0.05%-2.5% by mass, optionally 0.05%-0.5%. This reduces the internal short circuits caused by sodium dendrite growth in the sodium secondary battery, thereby improving the cycle performance of the sodium secondary battery.

[0012] In some embodiments, the volume ratio of the linear ether to the cyclic ether is (60:40) to (90:10). This reduces the internal short circuit caused by sodium dendrite growth in the sodium secondary battery, thereby improving the cycle performance of the sodium secondary battery.

[0013] In some embodiments, at least a portion of the surface of the first fast ion conductor is coated with a carbon-coated material, wherein the carbon-coated material accounts for 1.5%-2.5% of the total mass of the first fast ion conductor. This reduces the internal shortness caused by sodium dendrite growth in the sodium secondary battery, thereby improving the cycle performance of the sodium secondary battery.

[0014] In some embodiments, the residual alkali in the second fast ion conductor accounts for 0.05%-2.5% by mass, optionally 0.05%-0.5%; and / or, at least a portion of the surface of the second fast ion conductor is covered with a carbon-coated material, the carbon-coated material accounting for 1.5%-2.5% by mass based on the total mass of the second fast ion conductor. This can reduce the internal shortness caused by sodium dendrite growth in sodium secondary batteries and improve the cycle performance of sodium secondary batteries.

[0015] In some embodiments, the mass ratio of the positive electrode active material to the second fast ion conductor is (80:20)-(95:5). This reduces the internal short circuit caused by sodium dendrite growth in the sodium secondary battery, thereby improving the cycle performance of the sodium secondary battery.

[0016] In some embodiments, the mass percentage of the first fast ion conductor is 1%-3% based on the total mass of the positive electrode active material layer. This reduces the internal shortness caused by sodium dendrite growth in the sodium secondary battery, thereby improving the cycle performance of the sodium secondary battery.

[0017] In some embodiments, based on the total mass of the positive electrode active material, the first fast ion conductor, and the second fast ion conductor, the sum of the masses of the first and second fast ion conductors accounts for 5%-20%; and / or, based on the total mass of the second layer, the mass percentage of the second fast ion conductor is 90%-95%; and / or, the volume average particle size Dv50 of the second fast ion conductor is 2.5μm-3.5μm. This can reduce the internal shortness caused by sodium dendrite growth in sodium secondary batteries and improve the cycle performance of sodium secondary batteries.

[0018] In some embodiments, the electronic conductivity of the positive electrode is greater than or equal to 25 μS·cm. -1 Therefore, the internal short circuit caused by the growth of sodium dendrites in sodium secondary batteries can be reduced, thereby improving the cycle performance of sodium secondary batteries.

[0019] In some embodiments, the sodium secondary battery satisfies one or more of the following conditions: the ionic conductivity of the electrolyte is greater than or equal to 9 mS·cm. -1 The linear ether comprises at least one of ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, polyethylene glycol dimethyl ether, ethylene glycol diethyl ether, ethylene glycol dibutyl ether, diethylene glycol dibutyl ether, or diethylene glycol diethyl ether; the cyclic ether comprises at least one of tetrahydrofuran, methyltetrahydrofuran, or 1,3-dioxane; the volume average particle size Dv50 of the first fast ion conductor is 2.5 μm-3.5 μm. Therefore, the internal shortness caused by sodium dendrite growth in sodium secondary batteries can be reduced, improving the cycle performance of sodium secondary batteries.

[0020] In some embodiments, the positive electrode active material includes one or more of sodium transition metal oxides and phosphate compounds; the sodium transition metal oxide includes Na a1 M1 b1 O c1 M1 includes one or more of Ti, V, Cr, Mn, Fe, Co, Ni, or Cu, with 0.67 ≤ a1 ≤ 1.2, 0.9 ≤ b1 ≤ 1.1, and 0.9 ≤ c1 ≤ 2.1; and / or, the phosphate compound includes Na. x3 R y3 P m O n Wherein, 2.5≤x3≤4.5, 1.5≤y3≤3.5, 2.5<m<4.5, 11.5≤n≤15.5, and R includes one or more of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Cr, Nb, Mo, In, Ga, Sn, Hf, Ta, W, or Pb; and / or, based on the total mass of the positive electrode active material layer, the mass percentage of the positive electrode active material is 77%-80%. This can reduce the internal shortness caused by the growth of sodium dendrites in sodium secondary batteries and improve the cycle performance of sodium secondary batteries.

[0021] In some embodiments, the positive electrode active material comprises a sodium transition metal oxide, wherein the sodium transition metal oxide comprises Na 1+x2 M1O 2+y2 Wherein, M1 includes one or more of Ti, V, Cr, Mn, Fe, Co, Ni or Cu, -0.1≤x2≤0.1, -0.1≤y2≤0.1, and the sodium secondary battery satisfies one or more of the following conditions: the areal density of the positive electrode active material layer is 8 mg / cm³. 2 -9.5mg / cm 2 The thickness of the positive electrode active material layer is 25 μm-33.9 μm; the areal density of the second layer is 0.5 mg / cm³. 2 -2mg / cm 2 The thickness of the second layer is 2.5 μm-11.7 μm; the total compaction density of the positive electrode active material layer and the second layer is 2.58 g / cm³. 3 -2.75g / cm 3 The compaction density of the positive electrode active material layer is 2.8 g / cm³. 3 -3.2g / cm 3 The compacted density of the second layer is 1.7 g / cm³. 3 -2g / cm 3Therefore, the internal short circuit caused by the growth of sodium dendrites in sodium secondary batteries can be reduced, thereby improving the cycle performance of sodium secondary batteries.

[0022] In some embodiments, the positive electrode active material comprises a phosphate compound, wherein the phosphate compound includes Na. x3 R y3 P m O n Wherein, 2.5≤x³≤4.5, 1.5≤y³≤3.5, 2.5<m<4.5, 11.5≤n≤15.5, and R includes one or more of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Cr, Nb, Mo, In, Ga, Sn, Hf, Ta, W, or Pb. The sodium secondary battery satisfies one or more of the following conditions: the areal density of the positive electrode active material layer is 8 mg / cm³. 2 -9.5mg / cm 2 The thickness of the positive electrode active material layer is 40 μm-55.9 μm; the areal density of the second layer is 0.5 mg / cm³. 2 -2mg / cm 2 The thickness of the second layer is 2.5 μm-11.7 μm; the total compaction density of the positive electrode active material layer and the second layer is 1.7 g / cm³. 3 -2g / cm 3 The compaction density of the positive electrode active material layer is 1.7 g / cm³. 3 -2g / cm 3 The compacted density of the second layer is 1.7 g / cm³. 3 -2g / cm 3 Therefore, the internal short circuit caused by the growth of sodium dendrites in sodium secondary batteries can be reduced, thereby improving the cycle performance of sodium secondary batteries.

[0023] In some embodiments, the sodium secondary battery includes a sodium metal battery. This reduces internal short circuits caused by sodium dendrite growth in the sodium secondary battery, thereby improving its cycle performance.

[0024] In some embodiments, the negative electrode includes a negative current collector and an interface modification layer disposed on at least one side of the negative current collector, the interface modification layer including a binder and a conductive agent. This can reduce the internal short circuits caused by sodium dendrite growth in the sodium secondary battery, thereby improving the cycle performance of the sodium secondary battery.

[0025] In a second aspect of this application, an electrical device is proposed, including the secondary battery described in the first aspect of this application.

[0026] Additional aspects and advantages of this application will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of this application. Attached Figure Description

[0027] Various other advantages and benefits will become apparent to those skilled in the art upon reading the detailed description of the preferred embodiments below. The accompanying drawings are for illustrative purposes only and are not intended to limit the scope of this application. Furthermore, the same reference numerals denote the same parts throughout the drawings. In the drawings:

[0028] Figure 1 is a schematic diagram of a battery according to one embodiment of this application.

[0029] Figure 2 is an exploded view of the battery according to one embodiment of this application shown in Figure 1.

[0030] Figure 3 is a schematic diagram of a battery module according to one embodiment of this application.

[0031] Figure 4 is a schematic diagram of a battery pack according to one embodiment of this application.

[0032] Figure 5 is an exploded view of the battery pack of one embodiment of this application shown in Figure 4.

[0033] Figure 6 is a schematic diagram of an electrical device in which a battery is used as a power source according to an embodiment of this application.

[0034] Explanation of reference numerals in the attached drawings: 1. Battery cell; 11. Housing; 12. Electrode assembly; 13. Cover plate; 2. Battery module; 3. Battery pack; 31. Upper casing; 32. Lower casing. Detailed Implementation

[0035] The embodiments of the technical solution of this application are described in detail below. The following embodiments are only used to illustrate the technical solution of this application more clearly, and are therefore only examples, and should not be used to limit the scope of protection of this application.

[0036] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0037] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0038] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0039] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0040] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0041] Currently, judging from market trends, the application of rechargeable batteries is becoming increasingly widespread. Rechargeable batteries are not only used in energy storage power systems such as hydropower, thermal power, wind power, and solar power plants, but also widely used in electric vehicles such as electric bicycles, electric motorcycles, and electric cars, as well as in military equipment and aerospace, among other fields.

[0042] Sodium-ion batteries, as a promising energy storage technology, have shown broad development prospects due to their abundant raw materials, low manufacturing costs, and strong temperature adaptability. However, due to their system characteristics, dendrites easily form on the negative electrode in sodium-ion batteries. Taking a negative electrodeless sodium metal battery as an example, during charging, sodium ions extracted from the positive electrode deposit on the undercoat of the negative electrode to form metallic sodium. The sodium metal deposition process is affected by factors such as the electrolyte, the separator, and the ion diffusion process of the positive electrode active material. Because the positive electrode active material has a strong oxidizing effect under high voltage, with the increase of the number of cycles, the side reactions on the surface of the positive electrode active material gradually increase, forming a gradually thickening CEI (Cathode Electrolyte Interface), which leads to a gradual decrease in the sodium ion insertion / extraction rate. At the same time, the side reactions can also cause pore blockage in the separator. In addition, the binding effect of the solvent in the electrolyte on sodium ions will also increase the resistance to sodium ions detaching from the solvent and depositing on the negative electrode. These reasons will cause uneven charge distribution of sodium ions during transport and deposition, and dendrites will gradually form on the negative electrode. The cumulative growth of these dendrites will cause a short circuit inside the battery (internal short), which will cause the battery capacity to deteriorate rapidly and greatly limit the cycle stability of sodium secondary batteries.

[0043] To suppress the internal short circuits caused by sodium dendrite growth and improve battery performance, various improvement strategies have emerged. Optimizing the electrolyte composition, especially the use of additives, is the most common method for improving the performance of sodium rechargeable batteries because it is simple, compatible with current industrial production lines, and is one of the most widely used methods. Most additives help form a stable SEI (Solid Electrolyte Interface) and further alleviate the sodium dendrite problem, such as introducing fluorine-containing ions or solvents that preferentially participate in SEI formation at the negative electrode. However, preferential participation in interface construction leads to inevitable and irreversible degradation, and the effect of a small amount of additive is limited. Secondly, the battery also experiences capacity decay during long-term cycling. Although adding additives alleviates this problem, it increases the manufacturing cost of sodium rechargeable batteries, and fluorine-containing or other functional groups (such as CF) can produce undesirable side reactions with sodium metal.

[0044] In the sodium secondary battery of this application embodiment, the positive electrode active material layer includes a first fast ion conductor, which includes one or more of sodium pyrophosphate, sodium manganese vanadium phosphate, and sodium titanium manganese phosphate. The first fast ion conductor has a high sodium ion diffusion coefficient, resulting in faster ion transport and more uniform charge distribution when sodium ions enter the electrolyte from the surface of the first fast ion conductor. This makes the morphology of the sodium metal deposition on the negative electrode smoother, achieving uniform sodium deposition and reducing the internal short circuit of the sodium secondary battery. Furthermore, the first fast ion conductor exhibits excellent stability, reducing the oxidative decomposition reaction of the electrolyte at the interface between the positive electrode and the electrolyte, thus reducing electrolyte consumption, CEI film growth, and the continuous occurrence of membrane blockage. More importantly, experiments have shown that when an internal short circuit occurs during the cycling process of the sodium secondary battery, the capacity of the first fast ion conductor hardly decreases. When using a solvent containing linear and cyclic ethers, the weak solvation effect of the cyclic ethers weakens the sodium ionization reaction. + The desolvation energy reduces the solvent's dependence on Na. + When sodium dendrites come into contact with the first fast ion conductor, they undergo an oxidation reaction to become sodium ions, and simultaneously, the Na+ in the solution... + Rapid embedding into the first fast ion conductor structure and the excellent reaction kinetics process reduce the consumption of active sodium at the interface, which can reduce the internal shortness caused by the growth of sodium dendrites in sodium secondary batteries and improve the cycle performance of sodium secondary batteries.

[0045] The sodium secondary battery disclosed in this application can be used in electrical devices that use batteries as a power source or in various energy storage systems that use batteries as energy storage elements. Electrical devices may include, but are not limited to, mobile phones, tablets, laptops, electric toys, power tools, electric vehicles, electric cars, ships, spacecraft, etc. Electric toys may include stationary or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys, etc. Spacecraft may include airplanes, rockets, space shuttles, and spacecraft, etc.

[0046] The first aspect of this application discloses a sodium secondary battery, the sodium secondary battery comprising a positive electrode, an electrolyte, and a negative electrode, wherein the positive electrode comprises a positive current collector and a positive active material layer disposed on at least one side of the positive current collector, the positive active material layer comprising a positive active material and a first fast ion conductor, the first fast ion conductor comprising one or more of sodium pyrophosphate, sodium manganese vanadium phosphate, and sodium titanium manganese phosphate; the electrolyte comprises a solvent, the solvent comprising linear ether and cyclic ether.

[0047] In the sodium secondary battery of this application embodiment, the positive electrode active material layer includes a first fast ion conductor, which includes one or more of sodium pyrophosphate, sodium manganese vanadium phosphate, and sodium titanium manganese phosphate. The first fast ion conductor has a high sodium ion diffusion coefficient, resulting in faster ion transport and more uniform charge distribution when sodium ions enter the electrolyte from the surface of the first fast ion conductor. This makes the morphology of the sodium metal deposition on the negative electrode smoother, achieving uniform sodium deposition and reducing the internal short circuit of the sodium secondary battery. Furthermore, the first fast ion conductor has excellent stability, reducing the oxidative decomposition reaction of the electrolyte at the interface between the positive electrode and the electrolyte, thus reducing electrolyte consumption, CEI film growth, and the continuous occurrence of membrane blockage. More importantly, experiments have shown that when an internal short circuit occurs during the cycling process of the sodium secondary battery, the capacity of the first fast ion conductor hardly decays. When using a solvent containing linear and cyclic ethers, the weak solvation effect of the cyclic ethers weakens the sodium ion short circuit. + The desolvation energy reduces the solvent's dependence on Na. + When the generated sodium dendrites come into contact with the first fast ion conductor, the short sodium dendrites inside undergo an oxidation reaction to become sodium ions, and at the same time, the Na in the solution... + Rapid embedding into the first fast ion conductor structure and the excellent reaction kinetics process reduce the consumption of active sodium at the interface, which can reduce the internal shortness caused by the growth of sodium dendrites in sodium secondary batteries and improve the cycle performance of sodium secondary batteries.

[0048] In summary, when the first fast ion conductor is used in conjunction with solvents containing linear and cyclic ethers, it facilitates the desolvation of sodium ions and the kinetics of their deposition on the negative electrode. It can form a protective layer on the surface of the positive electrode active material layer, effectively improving the long-cycle stability of sodium secondary batteries. At the same time, due to the reduction of internal short circuits and side reactions, the storage performance and gas generation performance of sodium secondary batteries are also improved.

[0049] It is understandable that the ether group of a linear ether is attached to two carbon atoms, which can be adjacent carbon atoms on the same carbon chain or carbon atoms on different carbon chains; the ether group of a cyclic ether is attached to the carbon atom of a cyclic structure to form a cyclic ether.

[0050] It is understood that, in the embodiments of this application, linear ethers and cyclic ethers in the electrolyte can be determined by the following methods:

[0051] Detection was performed by gas chromatography-mass spectrometry (GC-MS).

[0052] It is understood that the material of the fast ion conductor in the embodiments of this application can be determined by the following method: the positive electrode active material layer of the positive electrode sheet is scraped into powder, the powder is dissolved in nitric acid, and the powder is determined by inductively coupled plasma (ICP) spectroscopy, for example, referring to standards YS / T1006.2-2014, GB / T 23367.2-2009, or YS / T 1028.5-2015. Specifically, according to the embodiments of this application, an inductively coupled plasma emission spectrometer can be used for measurement.

[0053] In some embodiments of this application, the sodium pyrophosphate salt includes Na. 4+a M 3+b (PO4) 2+c (P2O7) 1+d Where M includes at least one of Fe, Co, Mn, or Ni, and -0.1≤a≤0.1, -0.1≤b≤0.1, -0.1≤c≤0.1, -0.1≤d≤0.1; as an example, a can be -0.1, -0.05, 0, 0.05, 0.1, or any two of the above values; b can be -0.1, -0.05, 0, 0.05, 0.1, or any two of the above values; c can be -0.1, -0.05, ... The range of 0, 0.05, 0.1 or any two of the above values, and d can be -0.1, -0.05, 0, 0.05, 0.1 or any two of the above values. Therefore, the sodium ion diffusion coefficient of the above sodium pyrophosphate salt is high. When sodium ions enter the electrolyte from the surface of the first fast ion conductor, the ion transport is faster and the charge distribution is more uniform. This makes the morphology of the sodium metal deposition on the negative electrode smoother, realizes uniform sodium deposition, and thus reduces the internal shortness of the sodium secondary battery.

[0054] In other embodiments of this application, the sodium pyrophosphate salt comprises Na4Fe3(PO4)2P2O7 (NFPP). NFPP is a fast ion conductor with excellent stability and electronic conductivity, which can improve the sodium ion conductivity and electronic conductivity of sodium secondary batteries. Furthermore, in a solvent environment containing linear ethers and cyclic ethers, NFPP can oxidize sodium dendrites to Na+. + It is embedded in the Na4M3(PO4)2P2O7 structure, which reduces the consumption of active sodium at the interface, can reduce the internal shortness caused by the growth of sodium dendrites in sodium secondary batteries, and improve the cycle performance of sodium secondary batteries.

[0055] In some embodiments of this application, the sodium manganese vanadium phosphate comprises Na 3+f Mn x V 2-x (PO4) 3+y1Where -0.1≤f≤0.1, 0≤x≤1, -0.1≤y1≤0.1; as an example, f can be -0.1, -0.05, 0, 0.05, 0.1 or any two of the above values; x can be 0, 0.1, 0.2, 0.3, 0.5, 0.7, 0.9, 1 or any two of the above values; y1 can be -0.1, -0.05, 0, 0.05, 0.1 or any two of the above values. Therefore, the sodium ion diffusion coefficient of the above-mentioned sodium manganese vanadium phosphate is high, and the ion transport generated when sodium ions enter the electrolyte from the surface of the first fast ion conductor is faster and the charge distribution is more uniform, so that the morphology of the sodium metal deposition on the negative electrode is smoother, and uniform sodium deposition is achieved, thereby reducing the internal shortness of the sodium secondary battery.

[0056] The sodium titanium manganese phosphate includes Na 3+x1 MnTi 1-x1 V x1 (PO4) 3+e Where 0 ≤ x1 ≤ 1, -0.1 ≤ e ≤ 0.1. As an example, x1 can be 0, 0.1, 0.2, 0.3, 0.5, 0.7, 0.9, 1 or any two of the above values, and e can be -0.1, -0.05, 0, 0.05, 0.1 or any two of the above values. Therefore, the sodium ion diffusion coefficient of the above-mentioned sodium titanium manganese phosphate is high, and the ion transport generated when sodium ions enter the electrolyte from the surface of the first fast ion conductor is faster and the charge distribution is more uniform. This makes the morphology of the sodium metal deposition on the negative electrode smoother, achieving uniform sodium deposition, thereby reducing the internal shortness of the sodium secondary battery.

[0057] In some embodiments of this application, the positive electrode further includes a second layer disposed on the side of the positive electrode active material layer facing away from the positive electrode current collector. The second layer includes a second fast ion conductor, which comprises one or more of sodium pyrophosphate, sodium manganese vanadium phosphate, and sodium titanium manganese phosphate. This further improves the electronic conductivity of the positive electrode. When the generated sodium dendrites come into contact with the second fast ion conductor, the short sodium dendrites undergo an oxidation reaction to become sodium ions, while the Na in the solution... + Rapid embedding into the second fast ion conductor structure and the excellent reaction kinetics process reduce the consumption of active sodium at the interface, which can reduce the internal shortness caused by the growth of sodium dendrites in sodium secondary batteries and improve the cycle performance of sodium secondary batteries.

[0058] It is understood that the positive electrode active material layer and the second layer in the embodiments of this application can be seen by using an ion-polished cross-sectional morphology (CP) image of the positive electrode sheet obtained by using a ZEISS Sigma300 scanning electron microscope.

[0059] The materials of the positive electrode active material, the first fast ion conductor, and the second fast ion conductor can be determined by the following methods:

[0060] Based on the CP diagram obtained above, the specific positions of the positive electrode active material layer and the second layer are determined. Then, powder is scraped sequentially along the direction from the second layer to the current collector, and the number of times the powder is scraped to the second layer and the positive electrode active material layer is recorded respectively. Then, according to the above-mentioned number of times the powder is scraped to the second layer and the positive electrode active material layer, the powder is scraped and sampled from the dried positive electrode sheet to obtain the upper layer powder (powder in the second layer). Then, the powder is scraped and sampled to obtain the bottom layer powder (positive electrode active material layer).

[0061] The upper and lower layer powders are dissolved in nitric acid and analyzed by inductively coupled plasma (ICP) spectroscopy, for example, referring to standards YS / T 1006.2-2014, GB / T 23367.2-2009, or YS / T 1028.5-2015. Specifically, according to the embodiments of this application, an inductively coupled plasma emission spectrometer can be used for measurement.

[0062] It is understandable that the chemical formulas of sodium pyrophosphate, sodium manganese vanadium phosphate, and sodium titanium manganese phosphate in the second fast ion conductor can be the same as those of the corresponding sodium pyrophosphate, sodium manganese vanadium phosphate, and sodium titanium manganese phosphate in the first fast ion conductor.

[0063] In some embodiments of this application, the mass percentage of residual alkali in the first fast ion conductor is 0.05%-2.5%. For example, it can be 0.05%-2.4%, 0.1%-2%, 0.5%-1.5%, 1%-2%, etc. Residual alkali refers to alkaline substances remaining on the surface of the positive electrode active material, such as sodium carbonate, sodium hydroxide, etc. Controlling the content of residual alkali in the first fast ion conductor within the above range can reduce the side reactions of the first fast ion conductor during charging and discharging, improve the kinetics of the first fast ion conductor, improve the electronic conductivity of the first fast ion conductor, reduce gas production in the sodium secondary battery, reduce the probability of reaction between the first fast ion conductor and the electrolyte, and accelerate the oxidation and dissolution of sodium dendrites when they are generated, thereby improving the cycle performance of the sodium secondary battery. In other embodiments of this application, the mass percentage of residual alkali in the first fast ion conductor is 0.05%-0.5%.

[0064] It is understood that, in the embodiments of this application, "the mass percentage of residual alkali in the first fast ion conductor" can be determined by the following method:

[0065] Test principle (acid-base titration):

[0066] A certain mass m of the first fast ion conductor powder was taken, and sodium bicarbonate and sodium carbonate in the positive electrode material were titrated with a standard hydrochloric acid solution. Using a pH electrode as the indicator electrode, the endpoint was determined by the abrupt change in potential, and the gas production volumes V1 and V2 corresponding to the pH abrupt change points were measured. Based on the titration endpoint, the titration volume of the standard solution was determined. The calculated masses of Na2CO3 and NaHCO3 were divided by the mass of the positive electrode active material, and the sodium ion mass content was used as the residual alkali content of the positive electrode active material. Na2CO3% = (V2 - V1) × C × 106 × 100n / 1000m NaHCO3% = V2 × C × 84 × n × 100 / 1000m + %=V2×C×23×n×100 / 1000m

[0067] Where C is the concentration of the hydrochloric acid standard solution (0.05 mol / L); m: sample mass; n: 100 mL / pipette volume; 106 is the molecular weight of Na2CO3; and 84 is the molecular weight of NaHCO3.

[0068] In some embodiments of this application, at least a portion of the surface of the first fast ion conductor is covered with a carbon coating material. Based on the total mass of the first fast ion conductor, the mass percentage of the carbon coating material is 1.5%-2.5%. For example, it could be 1.5%-2.4%, 1.7%-2.3%, 2%-2.2%, etc. The carbon coating material refers to a carbon-based substance formed on at least a portion of the surface of the first fast ion conductor. The carbon coating layer can improve the conductivity of the first fast ion conductor and can, to a certain extent, isolate the electrolyte from the first fast ion conductor, reducing side reactions. It can also improve the diffusion coefficient of sodium ions, reducing internal short circuits in sodium secondary batteries containing it. Furthermore, when internal short circuits occur, the generated sodium dendrites, upon contact with the first fast ion conductor, rapidly oxidize to Na on the surface of the first fast ion conductor. + It is embedded in the first fast ion conductor structure, which reduces the consumption of active sodium at the interface, can reduce the internal shortness caused by the growth of sodium dendrites in sodium secondary batteries, and improve the cycle performance of sodium secondary batteries.

[0069] It is understood that in the embodiments of this application, the preparation of the first fast ion conductor containing carbon coating material is a method known in the art. For example, carbon sources such as oleic acid, dopamine, and resin can be used to form a uniform carbon coating material on the surface of the first fast ion conductor during heat treatment.

[0070] In some embodiments of this application, the residual alkali content in the second fast ion conductor is 0.05%-2.5% by mass. For example, it can be 0.05%-2.4%, 0.1%-2%, 0.5%-1.5%, 1%-2%, etc. Residual alkali refers to alkaline substances remaining on the surface of the positive electrode active material, such as sodium carbonate and sodium hydroxide. Controlling the residual alkali content in the second fast ion conductor within the above range can reduce side reactions during charging and discharging, improve the kinetics of the second fast ion conductor, increase the electronic conductivity of the second fast ion conductor, reduce gas production in the sodium secondary battery, reduce the probability of reaction between the second fast ion conductor and the electrolyte, and accelerate the oxidative dissolution of sodium dendrites when they are generated, thereby improving the cycle performance of the sodium secondary battery. In other embodiments of this application, the residual alkali content in the second fast ion conductor is 0.05%-0.5% by mass.

[0071] It is understandable that the method for determining the mass percentage of residual alkali in the second fast ion conductor is similar to that for determining the mass percentage of residual alkali in the first fast ion conductor, and will not be elaborated further.

[0072] In some embodiments of this application, at least a portion of the surface of the second fast ion conductor is covered with a carbon coating material. Based on the total mass of the second fast ion conductor, the mass percentage of the carbon coating material is 1.5%-2.5%. For example, it could be 1.5%-2.4%, 1.7%-2.3%, 2%-2.2%, etc. The carbon coating material refers to a carbon-based substance formed on at least a portion of the surface of the second fast ion conductor. The carbon coating layer can improve the conductivity of the second fast ion conductor and can, to a certain extent, isolate the electrolyte from the second fast ion conductor, reducing side reactions. It can also improve the diffusion coefficient of sodium ions, reducing internal short circuits in sodium secondary batteries containing it. Furthermore, when internal short circuits occur, the generated sodium dendrites, upon contact with the second fast ion conductor, rapidly oxidize to Na on the surface of the second fast ion conductor. + It is embedded in the second fast ion conductor structure, which reduces the consumption of active sodium at the interface, can reduce the internal shortness caused by the growth of sodium dendrites in sodium secondary batteries, and improve the cycle performance of sodium secondary batteries.

[0073] It is understandable that the preparation method of the carbon coating material on the surface of the second fast ion conductor is similar to that of the first fast ion conductor, and will not be described in detail here.

[0074] In some embodiments of this application, the volume ratio of the linear ether to the cyclic ether is (60:40)-(90:10), for example, 60:40, 70:30, 80:20, 90:10, etc. Controlling the volume ratio of the linear ether to the cyclic ether within the above range ensures that the solvent can both dissolve the sodium salt and have a weak solvation effect on sodium ions. This reduces both the excessive solvation effect on sodium ions caused by too much linear ether and the weak solubility of sodium salts caused by too little linear ether. Furthermore, it facilitates the rapid oxidation of sodium dendrites to Na4M3(PO4)2P2O7 on the surface when sodium dendrites are formed. + It is embedded in the Na4M3(PO4)2P2O7 structure, which reduces the consumption of active sodium at the interface, reduces the internal shortness caused by the growth of sodium dendrites in sodium secondary batteries, and can further improve the cycle performance of sodium secondary batteries.

[0075] It is understood that the "volume ratio of linear ethers to cyclic ethers" is a well-known definition in the art and can be determined using methods known in the art, such as the following methods:

[0076] Detection was performed using gas chromatography-mass spectrometry (GC-MS). After disassembling the sodium secondary battery, a portion of the electrolyte was taken and separated using gas chromatography. The separated substances were then detected and identified by mass spectrometry. Ether solvents were separated in gas chromatography and then analyzed qualitatively and quantitatively based on the mass-to-charge ratio of their fragment ions. Qualitative analysis was performed based on the retention time and characteristic ions of each component in the mass spectrum, while quantitative analysis was performed based on the peak area and the proportion of each solvent.

[0077] In some embodiments of this application, the electronic conductivity of the positive electrode is greater than or equal to 25 μS·cm. -1 For example, it could be 25 μS·cm -1 -80μS·cm -1 30 μS·cm -1 -70μS·cm -1 40 μS·cm -1 -60μS·cm -1 When the electronic conductivity of the positive electrode is within the above range, the positive electrode exhibits good conductivity. Furthermore, the weak solvation effect of linear and cyclic ether solvents helps reduce the desolvation process of sodium ions, thereby improving the Na+ conductivity. + The dynamics of ions, and the excellent combination of electron and ion dynamics, are essential to reduce the consumption of active sodium caused by side reactions during internal short circuits, thereby improving the cycle performance of secondary batteries.

[0078] It is understood that the "electronic conductivity of the positive electrode" is a well-known definition in the art and can be measured using methods known in the art, such as the following methods:

[0079] Cut the dried positive electrode film into small round pieces with a diameter of 3mm from the left, center, and right sides of the positive electrode sheet. Turn on the Yuaneng Technology electrode resistance meter, place it at the appropriate position of the "probe" on the electrode resistance meter, input the thickness parameter of the electrode, click the "start" button, and wait for the reading to stabilize before taking the reading. Test two positions for each small round piece, and finally calculate the average of the six measurements, which is the electronic conductivity of the electrode.

[0080] In some embodiments of this application, the mass ratio of the positive electrode active material to the second fast ion conductor is (80:20)-(95:5). For example, the mass ratio of the positive electrode active material to the second fast ion conductor can be 80:20, 85:15, 90:10, 95:5, etc. By controlling the mass ratio of the positive electrode active material to the second fast ion conductor within the above range, the second fast ion conductor can better cooperate with the positive electrode active material and solvents containing linear ethers and cyclic ethers, which helps the desolvation of sodium ions and the kinetics of deposition on the negative electrode. It can form a protective layer on the surface of the positive electrode active material layer, which can effectively improve the long-cycle stability of sodium secondary batteries. At the same time, due to the reduction of internal short circuits and side reactions, the storage performance and gas generation performance of sodium secondary batteries are also improved.

[0081] It is understood that the "mass ratio of the positive electrode active material to the second fast ion conductor" is a well-known definition in the art and can be determined using methods known in the art, such as the following methods:

[0082] First, the thickness of the positive electrode active material layer and the second layer of the electrode sheet are photographed. This can be seen by obtaining the ion-polished cross-sectional morphology (CP) of the positive electrode sheet using a ZEISS Sigma300 scanning electron microscope.

[0083] The mass of the positive electrode active material and the second fast ion conductor can be determined by the following methods:

[0084] Based on the CP diagram obtained above, the specific positions of the positive electrode active material layer and the second layer are determined. Then, powder is scraped sequentially along the direction from the second layer to the current collector, and the number of times powder is scraped to the second layer and the positive electrode active material layer are recorded respectively. Then, according to the above-mentioned number of times powder is scraped to the second layer and the positive electrode active material layer, the powder of the dried positive electrode sheet is sampled to obtain the upper layer powder and its mass (powder in the second layer) is weighed. Then, the powder is scraped and sampled to obtain the mass of the bottom layer powder (positive electrode active material layer). By calculation, the mass ratio of the positive electrode active material and the second fast ion conductor in the second layer can be obtained.

[0085] In some embodiments of this application, the ionic conductivity of the electrolyte is greater than or equal to 9 mS·cm. -1 For example, the ionic conductivity of the electrolyte can be 9 mS·cm. -1 -60mS·cm -1 10mS·cm -1 -50mS·cm -1 20mS·cm -1 -40mS·cm -1 30mS·cm -1 -35mS·cm -1 By controlling the ionic conductivity of the electrolyte within the above range, the kinetics of sodium ion desolvation can be facilitated, a protective layer can be formed on the surface of the positive electrode active material layer, which can effectively improve the long-cycle stability of sodium secondary batteries. At the same time, due to the reduction of internal short circuits and side reactions, the storage performance and gas generation performance of sodium secondary batteries are also improved.

[0086] In some embodiments of this application, the linear ether includes at least one of ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, polyethylene glycol dimethyl ether, ethylene glycol diethyl ether, ethylene glycol dibutyl ether, diethylene glycol dibutyl ether, or diethylene glycol diethyl ether. The above-mentioned linear ethers have good solubility for sodium salts and are compatible with various cyclic ethers. When used as a solvent in combination with cyclic ethers, the solvent's binding effect on sodium ions is weakened (weak solvation), making it easier for sodium ions to deposit from the solvent onto the negative electrode, reducing the formation of sodium dendrites. Furthermore, the probability of the above-mentioned linear ethers undergoing side reactions with the positive electrode active material, the first fast ion conductor, and the second fast ion conductor is low, which can further improve the cycle performance of sodium secondary batteries.

[0087] In some embodiments of this application, the cyclic ether comprises at least one of tetrahydrofuran (THF), methyltetrahydrofuran, or 1,3-dioxane. The aforementioned cyclic ethers exhibit weak solvation of sodium ions, meaning that when sodium ions are deposited onto the negative electrode, the binding effect on the sodium ions is weak, making it easier for sodium ions to deposit from the solvent onto the negative electrode, reducing the formation of sodium dendrites. Furthermore, the probability of the aforementioned linear ethers undergoing side reactions with the positive electrode active material, the first fast ion conductor, and the second fast ion conductor is low. When sodium dendrites are formed, they can facilitate the rapid oxidation of sodium dendrites to Na on the surfaces of the first and second fast ion conductors. + It is embedded in the structure of the first fast ion conductor and the second fast ion conductor, which reduces the consumption of active sodium at the interface, reduces the internal shortness caused by the growth of sodium dendrites in sodium secondary batteries, and can further improve the cycle performance of sodium secondary batteries.

[0088] In some embodiments of this application, the volume average particle size Dv50 of the first fast ion conductor is 2.5 μm-3.5 μm. For example, it can be 2.5 μm-3.4 μm, 2.8 μm-3.2 μm, 2.9 μm-3 μm, etc. By controlling the volume average particle size Dv50 of the first fast ion conductor within the above range, the ion transport generated when sodium ions enter the electrolyte from the surface of the first fast ion conductor is faster and the charge distribution is more uniform. This results in a smoother morphology of the sodium metal deposition on the negative electrode, achieving uniform sodium deposition, thereby reducing the internal shortness of the sodium secondary battery and improving the cycle performance of the sodium secondary battery.

[0089] It is understood that the volume average particle size Dv50 refers to the particle size corresponding to a cumulative volume distribution percentage of 50%. The volume average particle size Dv50 of Na4M3(PO4)2P2O7 can be determined using methods known in the art, for example, by the following methods:

[0090] The volume average particle size Dv50 of Na4M3(PO4)2P2O7 was tested using a laser particle size analyzer (e.g., Malvern Master Sizer 3000) in accordance with standard GB / T 19077-2016.

[0091] In some embodiments of this application, the volume average particle size Dv50 of the second fast ion conductor is 2.5 μm-3.5 μm. For example, it can be 2.5 μm-3.4 μm, 2.8 μm-3.2 μm, 2.9 μm-3 μm, etc. By controlling the volume average particle size Dv50 of the second fast ion conductor within the above range, the ion transport generated when sodium ions enter the electrolyte from the surface of the second fast ion conductor is faster and the charge distribution is more uniform. This results in a smoother morphology of the sodium metal deposition on the negative electrode, achieving uniform sodium deposition, thereby reducing the internal shortness of the sodium secondary battery and improving the cycle performance of the sodium secondary battery.

[0092] In some embodiments of this application, the positive electrode active material includes at least one of sodium transition metal oxides, polyanionic compounds, and Prussian blue sodium compounds, as well as their respective modified compounds. Using at least one of these materials in conjunction with a first fast ion conductor and a second fast ion conductor can further reduce the formation of sodium dendrites and improve the cycle performance of sodium secondary batteries.

[0093] As an example, the positive electrode active material may include at least one of the following materials: sodium transition metal oxides, polyanionic compounds, and Prussian blue sodium compounds, and at least one of their respective modified compounds. However, this application is not limited to these materials, and other conventional materials that can be used as positive electrode active materials for batteries may also be used. The modified compounds of the above materials may be for doping modification and / or surface coating modification of the materials.

[0094] In some embodiments of this application, the sodium transition metal oxide includes Na. a1 M1 b1 O c1 Where M1 includes one or more of Ti, V, Cr, Mn, Fe, Co, Ni, or Cu, and 0.67≤a1≤1.2, 0.9≤b1≤1.1, 0.9≤c1≤2.1. As an example, a1 can be 0.67, 0.7, 0.8, 0.9, 1, 1.1, 1.2, or any range of two of these values; b can be 0.9, 0.95, 1, 1.05, 1.1, or any range of two of these values; and c can be 0.9, 1, 1.2, 1.4, 1.5, 1.7, 1.9, 2, 2.1, or any range of two of these values.

[0095] In some embodiments of this application, the polyanionic compound may be a compound containing sodium ions, transition metal ions, or a tetrahedral (YO4) structure. n1- A class of compounds with anionic units. The transition metal may include at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce; Y may include at least one of P, S, and Si; n1 represents (YO4). n1- The price state.

[0096] In some embodiments of this application, the polyanionic compound may also be a sodium ion, transition metal ion, or tetrahedral (YO4) compound. n1- A class of compounds containing anionic units and halide anions. Transition metals may include at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce; Y may include at least one of P, S, and Si, and n1 represents (YO4). n1- The valence state of halogens can include at least one of F, Cl, and Br.

[0097] In some embodiments of this application, the polyanionic compound may also be a tetrahedral compound containing sodium ions (YO4). n1- Anionic unit, polyhedral unit (ZO) y ) m1+And a class of compounds with optional halide anions. M may include at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr and Ce, Y may include at least one of P, S and Si, and n1 represents (YO4). n1- The valence state, Z represents the transition metal, m1 represents (ZO) y ) m1+ The valence state of halogens can include at least one of F, Cl, and Br.

[0098] As an example, polyanionic compounds can satisfy the chemical formulas NaFePO4, Na3V2(PO4)3 (sodium vanadium phosphate, abbreviated as NVP), Na4Fe3(PO4)2(P2O7), NaM'PO4F (M' includes at least one of V, Fe, Mn and Ni), and Na3(VO y1 )2(PO4)2F 3-2y1 At least one of (0≤y1≤1).

[0099] In some embodiments of this application, Prussian blue compounds may be compounds containing sodium ions, transition metal ions, and cyanide ions (CN). - A class of compounds. Transition metals may include at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce.

[0100] As an example, Prussian blue compounds can satisfy the chemical formula Na a Me b Me' c (CN)6, wherein Me and Me' each independently include at least one of Ni, Cu, Fe, Mn, Co, and Zn, 0 < a ≤ 2, 0 < b < 1, and 0 < c < 1.

[0101] During the charging and discharging process of a battery, sodium (Na) undergoes insertion / extraction and consumption, resulting in varying molar Na content at different discharge states. In the examples of positive electrode active materials in this application, the molar Na content refers to the initial state of the material, i.e., the state before material addition. After charge-discharge cycles, the molar Na content changes when the positive electrode active material is applied to the battery system.

[0102] In the examples of positive electrode active materials for sodium-ion batteries in this application, the molar content of O is only a theoretical value. Oxygen release from the crystal lattice will cause changes in the molar content of oxygen, and the actual molar content of O will fluctuate.

[0103] In other embodiments of this application, the positive electrode active material includes one or more of sodium transition metal oxides and phosphate compounds; the sodium transition metal oxide includes Na a1 M1 b1O c1 Where M1 includes one or more of Ti, V, Cr, Mn, Fe, Co, Ni, or Cu, and 0.67≤a1≤1.2, 0.9≤b1≤1.1, 0.9≤c1≤2.1. As an example, a1 can be 0.67, 0.7, 0.8, 0.9, 1, 1.1, 1.2, or any range of two of these values; b can be 0.9, 0.95, 1, 1.05, 1.1, or any range of two of these values; and c can be 0.9, 1, 1.2, 1.4, 1.5, 1.7, 1.9, 2, 2.1, or any range of two of these values.

[0104] In other embodiments of this application, the phosphate compound includes Na. x3 R y3 P m O n Wherein, 2.5≤x3≤4.5, 1.5≤y3≤3.5, 2.5<m<4.5, 11.5≤n≤15.5, and R includes one or more of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Cr, Nb, Mo, In, Ga, Sn, Hf, Ta, W, or Pb. As an example, x3 can be 2.5-4.4, 3-4, 3.5-3.8, etc.; y3 can be 1.5-3.4, 2-3, 2.5-2.8, etc.; m can be 2.6-4.4, 3-4, 3.5-3.8, etc.; and n can be 11.5-15.4, 12-15, 13-14, etc.

[0105] It is understandable that phosphate compounds can be compounds containing phosphate and sodium elements.

[0106] The aforementioned positive electrode active material has high capacity and strong stability. However, it exhibits strong oxidation under high pressure. As the number of cycles increases, the side reactions on the surface of the positive electrode active material gradually increase, forming a gradually thickening CEI (Cathode Electrolyte Interface) film. This leads to a gradual decrease in the sodium ion insertion / extraction rate. Simultaneously, the side reactions can also cause pore blockage in the separator. These problems are particularly prominent during cycling. Therefore, when the aforementioned positive electrode active material is used in conjunction with a first fast ion conductor and solvents containing linear and cyclic ethers, it further facilitates the desolvation of sodium ions and the kinetics of their deposition on the negative electrode. This can form a protective layer on the surface of the positive electrode active material layer, further improving the long-cycle stability of sodium secondary batteries. At the same time, due to the reduction of internal short circuits and side reactions, the storage performance and gas generation performance of sodium secondary batteries are also improved.

[0107] In some embodiments of this application, the positive electrode active material comprises a sodium transition metal oxide, and the sodium transition metal oxide includes Na. 1+x2 M1O 2+y2 Wherein, M1 includes one or more of Ti, V, Cr, Mn, Fe, Co, Ni or Cu, -0.1≤x2≤0.1, -0.1≤y2≤0.1, and the sodium secondary battery satisfies one or more of the following conditions:

[0108] The total compaction density of the positive electrode active material layer and the second layer is 2.58 g / cm³. 3 -2.75g / cm 3 For example, it could be 2.58 g / cm³ 3 -2.74g / cm 3 2.6g / cm 3 -2.7g / cm 3 2.65g / cm 3 -2.69g / cm 3 By controlling the total compaction density of the positive electrode active material layer and the second layer within the above range, good contact can be achieved between the powder components of the positive electrode active material layer. At the same time, the electrolyte can easily wet the electrode sheet, thereby giving the electrode sheet excellent ionic and electronic conductivity in the electrolyte. This helps to facilitate the rapid and uniform insertion and extraction of sodium ions, reduces the internal shortness caused by the growth of sodium dendrites in sodium secondary batteries, and improves the cycle performance of sodium secondary batteries.

[0109] It can be understood that "the total compaction density of the positive electrode active material layer and the second layer" is a well-known definition in the art and can be measured using methods known in the art, for example, the following methods:

[0110] According to an embodiment of this application, the total compaction density PD of the positive electrode active material layer and the second layer is determined by measuring the total mass (g / cm³) of the positive electrode active material layer and the second layer per unit area on one side. 2 The total compaction density PD of the positive electrode active material layer and the second layer is determined by the total thickness (cm) of the single-sided positive electrode active material layer and the second layer (number of sampling points > 14). Specifically, the total compaction density PD of the positive electrode active material layer and the second layer is equal to the total mass (g / cm³) of the single-sided positive electrode active material layer and the second layer per unit area. 2 ) / Total thickness (cm) of the positive electrode active material layer and the second layer.

[0111] The compaction density of the positive electrode active material layer is 2.8 g / cm³. 3 -3.2g / cm 3 For example, it could be 2.8 g / cm³. 3 -3.19g / cm 3 2.9g / cm 3 -3.1g / cm3 3g / cm 3 -3.1g / cm 3 By controlling the compaction density of the positive electrode active material layer within the above range, the energy density and cycle performance of sodium secondary batteries can be balanced. This can reduce the internal shortness caused by the growth of sodium dendrites in sodium secondary batteries and improve their cycle performance.

[0112] The compacted density of the second layer is 1.7 g / cm³. 3 -2g / cm 3 For example, it could be 1.7 g / cm³ 3 -1.9g / cm 3 1.8g / cm 3 -1.9g / cm 3 By controlling the compaction density of the second layer within the above range, the sodium secondary battery can achieve a balance between energy density and cycle performance, and can also prevent the formation of sodium dendrites in the positive electrode active material layer. When sodium dendrites are formed, the second layer with the above-mentioned areal density can also work with the solvent to rapidly oxidize the sodium dendrites to Na on the surfaces of the first and second fast ion conductors. + By embedding it into the structure of the first fast ion conductor and the second fast ion conductor, the internal shortness caused by the growth of sodium dendrites in sodium secondary batteries can be reduced, thereby improving the cycle performance of sodium secondary batteries.

[0113] It can be understood that "the compaction density of the positive electrode active material layer and the compaction density of the second layer" are well-known definitions in the art and can be measured using methods well-known in the art, for example, the following methods:

[0114] According to embodiments of this application, the compaction density PD of the positive electrode film is determined by measuring the mass (g / cm³) of the positive electrode film per unit area on one side. 2 The density of the positive electrode film (PD1) is determined by the thickness of the positive electrode film on one side (cm) (number of sampling points > 14). Specifically, the compaction density of the positive electrode film PD1 = the mass of the positive electrode film per unit area on one side (g / cm³). 2 ) / Positive electrode film thickness (cm).

[0115] The thickness of the positive electrode active material layer and the second layer of the electrode sheet can be observed by using a ZEISS Sigma300 scanning electron microscope to obtain the ion-polished cross-sectional morphology (CP) image of the positive electrode sheet. Based on the obtained CP image, the specific locations of the positive electrode active material layer and the second layer are determined. Then, on a positive electrode sheet of a fixed area, powder is scraped sequentially along the direction from the second layer to the current collector, and the number of times the powder is scraped to the second layer is recorded. Then, according to the above-mentioned number of times the powder is scraped to the second layer, the powder of the dried positive electrode sheet is sampled to obtain the upper layer powder and its mass (powder in the second layer) is weighed. The compaction density PD2 of the second layer is calculated as the mass of the second layer per unit area on one side (g / cm³).2 The mass of the positive electrode active material layer can be obtained from the mass difference between the positive electrode sheet and the second layer. Therefore, the compaction density PD3 of the positive electrode active material layer can be calculated as: PD3 = mass of one side of the positive electrode active material layer per unit area (g / cm²). 2 ) / Thickness of the positive electrode active material layer (cm).

[0116] The areal density of the positive electrode active material layer is 8 mg / cm³. 2 -9.5mg / cm 2 For example, it could be 8 mg / cm³ 2 -9.4mg / cm 2 8.2 mg / cm 2 -9.3mg / cm 2 8mg / cm 2 -9mg / cm 2 8.5 mg / cm 2 -8.8mg / cm 2 By controlling the areal density of the positive electrode active material layer within the above range, the sodium secondary battery can balance energy density and cycle performance. Furthermore, sodium dendrites are less likely to form in the positive electrode active material layer, which can reduce the internal shortness caused by the growth of sodium dendrites in the sodium secondary battery and improve the cycle performance of the sodium secondary battery.

[0117] The thickness of the positive electrode active material layer is 25μm-33.9μm, for example, it can be 25μm-33.5μm, 26μm-33μm, 27μm-32μm, 28μm-31μm, 29μm-30μm, etc. Controlling the thickness of the positive electrode active material layer within the above range allows the sodium secondary battery to balance energy density and cycle performance. In addition, sodium dendrites are less likely to form in the positive electrode active material layer, which can reduce the internal shortness caused by the growth of sodium dendrites in the sodium secondary battery and improve the cycle performance of the sodium secondary battery.

[0118] The areal density of the second layer is 0.5 mg / cm³. 2 -2mg / cm 2 For example, it could be 0.5 mg / cm³. 2 -1.9mg / cm 2 1mg / cm 2 -1.5mg / cm 2 1.2 mg / cm 2 -1.3mg / cm 2 By controlling the areal density of the second layer within the above range, the sodium secondary battery can achieve a balance between energy density and cycle performance, while also preventing the formation of sodium dendrites. When sodium dendrites do form, the second layer with the aforementioned areal density can also interact with the solvent to rapidly oxidize the sodium dendrites to Na on the surface of the second fast ion conductor.+ It is embedded in the second fast ion conductor structure, which can reduce the internal shortness caused by the growth of sodium dendrites in sodium secondary batteries and improve the cycle performance of sodium secondary batteries.

[0119] It can be understood that "the areal density of the positive electrode active material layer and the areal density of the second layer" are well-known definitions in the art and can be measured using methods well-known in the art, for example, the following methods:

[0120] First, the thickness of the positive electrode active material layer and the second layer of the electrode sheet are photographed. This can be seen by obtaining the ion-polished cross-sectional morphology (CP) of the positive electrode sheet using a ZEISS Sigma300 scanning electron microscope.

[0121] Then, the mass of the positive electrode active material layer and the second layer can be measured using the following method:

[0122] Based on the CP diagram obtained above, the specific positions of the positive electrode active material layer and the second layer are determined. A positive electrode sheet with a fixed area is taken, and powder is scraped sequentially along the direction from the second layer to the current collector. The number of times powder is scraped to the second layer and the positive electrode active material layer is recorded respectively. Then, according to the above-mentioned number of times powder is scraped to the second layer and the positive electrode active material layer, the powder of the dried positive electrode sheet is sampled to obtain the upper layer powder and its mass (powder in the second layer). Then, the powder is scraped and sampled to obtain the mass of the bottom layer powder (positive electrode active material layer). Using the formula: material surface density = mass of active material / electrode sheet area, the surface density of the positive electrode active material layer and the surface density of the second layer can be obtained.

[0123] The thickness of the second layer is 2.5μm-11.7μm, for example, it can be 2.5μm-11.5μm, 3μm-11μm, 4μm-10μm, 5μm-9μm, 6μm-8μm, etc. Controlling the thickness of the second layer within the above range allows the sodium secondary battery to balance energy density and cycle performance, and can also prevent the formation of sodium dendrites. When sodium dendrites are formed, the second layer of the above thickness can also work with the solvent to rapidly oxidize the sodium dendrites to Na on the surface of the second fast ion conductor. + It is embedded in the second fast ion conductor structure, which can reduce the internal shortness caused by the growth of sodium dendrites in sodium secondary batteries and improve the cycle performance of sodium secondary batteries.

[0124] It can be understood that "the thickness of the positive electrode active material layer and the thickness of the second layer" are well-known definitions in the art and can be measured using methods well-known in the art, for example, the following methods:

[0125] According to the example of this application, the thickness of the positive electrode active material layer and the second layer of the electrode sheet is then photographed, and can be measured by using a ZEISS Sigma300 scanning electron microscope to obtain the ion-polished cross-sectional morphology (CP) image of the positive electrode sheet.

[0126] In other embodiments of this application, the positive electrode active material comprises a sodium transition metal oxide, wherein the sodium transition metal oxide comprises Na 1+x2 M1O 2+y2 Wherein, M1 includes one or more of Ti, V, Cr, Mn, Fe, Co, Ni or Cu, -0.1≤x2≤0.1, -0.1≤y2≤0.1, and the sodium secondary battery satisfies one or more of the following conditions:

[0127] The total compaction density of the positive electrode active material layer and the second layer is 1.7 g / cm³. 3 -2g / cm 3 For example, it could be 1.7 g / cm³ 3 -1.95g / cm 3 1.75g / cm 3 -1.9g / cm 3 1.8g / cm 3 -1.85g / cm 3 By controlling the total compaction density of the positive electrode active material layer and the second layer within the above range, good contact can be achieved between the component powders of the positive electrode active material layer and the second layer. At the same time, the electrolyte can easily wet the electrode, thereby giving the electrode excellent ionic conductivity and electronic conductivity in the electrolyte. This helps the rapid and uniform insertion and extraction of sodium ions, reduces the internal shortness caused by the growth of sodium dendrites in sodium secondary batteries, and improves the cycle performance of sodium secondary batteries.

[0128] The compaction density of the positive electrode active material layer is 1.7 g / cm³. 3 -2g / cm 3 For example, it could be 1.75 g / cm³. 3 -1.9g / cm 3 1.8g / cm 3 -1.85g / cm 3 By controlling the compaction density of the positive electrode active material layer within the above range, the energy density and cycle performance of sodium secondary batteries can be balanced. This can reduce the internal shortness caused by the growth of sodium dendrites in sodium secondary batteries and improve their cycle performance.

[0129] The compacted density of the second layer is 1.7 g / cm³. 3 -2g / cm 3 For example, it could be 1.7 g / cm³ 3 -1.9g / cm 3 1.8g / cm 3 -1.9g / cm 3By controlling the compaction density of the second layer within the above range, the sodium secondary battery can achieve a balance between energy density and cycle performance, while also preventing the formation of sodium dendrites in the positive electrode active material layer. When sodium dendrites do form, the compacted second layer can also interact with the solvent to rapidly oxidize the sodium dendrites to Na on the surface of the second fast ion conductor. + It is embedded in the second fast ion conductor structure, which can reduce the internal shortness caused by the growth of sodium dendrites in sodium secondary batteries and improve the cycle performance of sodium secondary batteries.

[0130] The areal density of the positive electrode active material layer is 8 mg / cm³. 2 -9.5mg / cm 2 For example, it could be 8 mg / cm³ 2 -9.4mg / cm 2 8.2 mg / cm 2 -9.3mg / cm 2 8mg / cm 2 -9mg / cm 2 8.5 mg / cm 2 -8.8mg / cm 2 By controlling the areal density of the positive electrode active material layer within the above range, the sodium secondary battery can balance energy density and cycle performance, and can reduce the internal shortness caused by the growth of sodium dendrites in the sodium secondary battery, thereby improving the cycle performance of the sodium secondary battery.

[0131] The thickness of the positive electrode active material layer is 40μm-55.9μm, for example, it can be 40μm-55.5μm, 45μm-55μm, 50μm-52μm, etc. Controlling the thickness of the positive electrode active material layer within the above range allows the sodium secondary battery to balance energy density and cycle performance, and can reduce the internal shortness caused by the growth of sodium dendrites in the sodium secondary battery, thereby improving the cycle performance of the sodium secondary battery.

[0132] The areal density of the second layer is 0.5 mg / cm³. 2 -2mg / cm 2 For example, it could be 0.5 mg / cm³. 2 -1.9mg / cm 2 1mg / cm 2 -1.5mg / cm 2 1.2 mg / cm 2 -1.3mg / cm 2 By controlling the areal density of the second layer within the above range, the sodium secondary battery can achieve a balance between energy density and cycle performance, while also preventing sodium dendrite formation. When sodium dendrites do form, the second layer with the aforementioned areal density can also interact with the solvent to rapidly oxidize the sodium dendrites to Na on the surface of the second fast ion conductor. +It is embedded in the second fast ion conductor structure, which can reduce the internal shortness caused by the growth of sodium dendrites in sodium secondary batteries and improve the cycle performance of sodium secondary batteries.

[0133] The thickness of the second layer is 2.5μm-11.7μm, for example, it can be 2.5μm-11.5μm, 3μm-11μm, 4μm-10μm, 5μm-9μm, 6μm-8μm, etc. Controlling the thickness of the second layer within the above range allows the sodium secondary battery to balance energy density and cycle performance, and can also prevent the formation of sodium dendrites. When sodium dendrites are formed, the second layer of the above thickness can also work with the solvent to rapidly oxidize the sodium dendrites to Na on the surface of the second fast ion conductor. + By embedding it into the structure of the second fast ion conductor, the internal shortness caused by the growth of sodium dendrites in sodium secondary batteries can be reduced, thereby improving the cycle performance of sodium secondary batteries.

[0134] In some embodiments of this application, based on the total mass of the positive electrode active material layer, the mass ratio of the positive electrode active material is 77%-80%, for example, it can be 77%-79%, 77%-78%, 78%-79%, etc. Controlling the mass ratio of the positive electrode active material within the above range can balance the energy density and cycle performance of sodium secondary batteries, reduce the internal shortness caused by the growth of sodium dendrites in sodium secondary batteries, and improve the cycle performance of sodium secondary batteries.

[0135] It is understood that "the mass percentage of the positive electrode active material based on the total mass of the positive electrode active material layer" is a well-known definition in the art and can be determined using methods known in the art, such as the following methods:

[0136] First, the positive electrode active material layer and its thickness are photographed. This can be seen by obtaining the ion-polished cross-sectional morphology (CP) of the positive electrode using a ZEISS Sigma300 scanning electron microscope.

[0137] Then, the mass of the positive electrode active material layer can be measured using the following method:

[0138] Based on the CP diagram obtained above, the specific positions of the positive electrode active material layer and the second layer are determined. A positive electrode sheet of fixed area is taken, and powder is scraped sequentially along the direction from the second layer to the current collector. The number of times powder is scraped to the second layer and the positive electrode active material layer is recorded respectively. Then, according to the above-mentioned number of times powder is scraped to the second layer and the positive electrode active material layer, a sample of the dried positive electrode sheet is taken to obtain the upper layer powder and its mass m2 (total mass of the second layer). Then, the powder is scraped and sampled to obtain the mass m1 of the bottom layer powder (total mass of the positive electrode active material layer). The bottom layer powder is dissolved with nitric acid, and the mass m3 of the positive electrode active material is determined by inductively coupled plasma (ICP) spectroscopy. Thus, the mass ratio m3 / m1 of the positive electrode active material can be obtained. For example, standards YS / T 1006.2-2014, GB / T 23367.2-2009, or YS / T 1028.5-2015 can be referenced. Specifically, according to the embodiments of this application, an inductively coupled plasma emission spectrometer can be used for measurement.

[0139] In some embodiments of this application, the mass percentage of the second fast ion conductor is 90%-95% based on the total mass of the second layer. For example, it could be 90%-94%, 91%-93%, 92%-93%, etc. Controlling the content of the second fast ion conductor in the second layer within these ranges allows the sodium secondary battery to balance energy density and cycle performance, while also preventing the formation of sodium dendrites. When sodium dendrites are formed, the second fast ion conductor in the second layer can also interact with the solvent to rapidly oxidize the sodium dendrites to Na on the surface of the second fast ion conductor. + It is embedded in the second fast ion conductor structure, which can reduce the internal shortness caused by the growth of sodium dendrites in sodium secondary batteries and improve the cycle performance of sodium secondary batteries.

[0140] It is understood that "the mass percentage of the second fast ion conductor based on the total mass of the second layer" is a well-known definition in the art and can be determined using methods known in the art, such as the following methods:

[0141] The thickness of the positive electrode active material layer and the second layer in the embodiments of this application can be seen by using a ZEISS Sigma300 scanning electron microscope to obtain the ion-polished cross-sectional morphology (CP) of the positive electrode sheet.

[0142] The material composition of the positive electrode active material and the second layer can be determined by the following methods:

[0143] Based on the CP diagram obtained above, the specific positions of the positive electrode active material layer and the second layer are determined. Then, powder is scraped sequentially along the direction from the second layer to the current collector, and the number of times the powder is scraped to the second layer is recorded. Then, according to the number of times the powder is scraped to the second layer, the powder is sampled from the dried positive electrode sheet to obtain the mass m2 of the upper layer powder (the total mass of the second layer).

[0144] The upper powder is dissolved in nitric acid, and the mass m1 of the second fast ion conductor is determined by inductively coupled plasma (ICP) spectroscopy. This yields the mass ratio m2 / m1 of the second fast ion conductor. For example, standards YS / T 1006.2-2014, GB / T 23367.2-2009, or YS / T 1028.5-2015 can be referenced. Specifically, according to the embodiments of this application, an inductively coupled plasma emission spectrometer can be used for measurement.

[0145] It is understandable that the choice of the first fast ion conductor and the second fast ion conductor are independent of each other; they can be the same or different.

[0146] In some embodiments of this application, the mass percentage of the first fast ion conductor is 1%-3% based on the total mass of the positive electrode active material layer. For example, it can be 1%-2.9%, 1.5%-2.5%, 2%-2.2%, etc. Controlling the mass percentage of the first fast ion conductor within the above range can reduce the slow sodium ion diffusion kinetics of the positive electrode sheet caused by too little first fast ion conductor, which leads to internal short circuits and subsequent consumption of active sodium. It can also reduce the capacity reduction of the positive electrode sheet caused by too much first fast ion conductor, which affects the energy density of the sodium secondary battery. Furthermore, it can reduce the internal short circuits caused by the growth of sodium dendrites in the sodium secondary battery and improve the cycle performance of the sodium secondary battery.

[0147] In some embodiments of this application, based on the total mass of the positive electrode active material, the first fast ion conductor, and the second fast ion conductor, the sum of the masses of the first fast ion conductor and the second fast ion conductor accounts for 5%-20%. For example, it can be 5%-19%, 10%-15%, 12%-13%, etc. Controlling the mass percentage of the first fast ion conductor and the second fast ion conductor within the above range is sufficient to improve the sodium ion conductivity and electronic conductivity of the entire positive electrode active material layer and the second layer. It can also reduce the impact on the capacity performance of the positive electrode sheet caused by excessive addition of the first fast ion conductor and the second fast ion conductor, thereby affecting the energy density of the sodium secondary battery. It can also reduce the internal shortness caused by the growth of sodium dendrites in the sodium secondary battery and improve the cycle performance of the sodium secondary battery.

[0148] Typically, a battery consists of a positive electrode, a negative electrode, an electrolyte, and a separator. During charging and discharging, active metal ions repeatedly insert and extract between the positive and negative electrodes. The electrolyte acts as a conductor of ions between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, primarily prevents short circuits while allowing ions to pass through.

[0149] In some embodiments of this application, the positive electrode includes a positive current collector and a positive active material layer disposed on at least one side of the positive current collector.

[0150] As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive active material layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0151] In some embodiments of this application, the positive current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer material substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0152] In some embodiments of this application, the positive electrode active material layer and the second layer may also each optionally include a binder independently. As an example, the binder may include at least one selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0153] In some embodiments of this application, the positive electrode active material layer and the second layer may also each optionally include a conductive agent. As an example, the conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0154] In some embodiments of this application, the positive electrode sheet can be prepared by dispersing the components used to prepare the positive electrode sheet, such as the positive electrode active material, conductive agent, binder, the first fast ion conductor, and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto a positive electrode current collector, and then performing processes such as drying and cold pressing to obtain the positive electrode active material layer. The preparation of the second layer is similar to the preparation method of the positive electrode active material layer, thus obtaining the positive electrode sheet.

[0155] In some embodiments of this application, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0156] In some embodiments of this application, when the battery is a sodium-ion battery, the electrolyte salt may include at least one of sodium hexafluorophosphate, sodium difluorooxalate borate, sodium tetrafluoroborate, sodium dioxalate borate, sodium perchlorate, sodium hexafluoroarsenate, sodium bis(fluorosulfonyl)imide, sodium trifluoromethanesulfonate, or sodium bis(trifluoromethanesulfonyl)imide.

[0157] In some embodiments of this application, the solvent may further include at least one selected from ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, ethylene glycol dimethyl ether, methyl ethyl sulfone, or diethyl sulfone.

[0158] In some embodiments of this application, the electrolyte may optionally include additives. For example, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0159] In some embodiments of this application, the negative electrode sheet includes a negative current collector and a negative active material layer disposed on at least one side of the negative current collector.

[0160] As an example, the negative electrode current collector has two surfaces opposite each other in its own thickness direction, and the negative electrode active material layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0161] In some embodiments of this application, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer material substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0162] In some embodiments of this application, the negative electrode active material may be a negative electrode active material known in the art for use in batteries. As an example, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and sodium titanate, etc. The silicon-based material may be selected from at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material may be selected from at least one of elemental tin, tin oxide compounds, and tin alloys. However, this application is not limited to these materials, and other conventional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0163] In some embodiments of this application, the negative electrode active material layer may optionally include a binder. The binder may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0164] In some embodiments of this application, the negative electrode active material layer may optionally include a conductive agent. The conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0165] In some embodiments of this application, the negative electrode active material layer may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0166] In some embodiments of this application, the negative electrode sheet can be prepared by dispersing the above-mentioned components for preparing the negative electrode sheet, such as negative electrode active material, conductive agent, binder and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto the negative electrode current collector, and after drying, cold pressing and other processes, the negative electrode sheet can be obtained.

[0167] In some embodiments of this application, the secondary battery includes a sodium metal battery. In the aforementioned secondary battery, the active sodium metal ions on the negative electrode are relatively reactive, and the deposition of sodium at the negative electrode is easily affected, resulting in uneven deposition and making it easier to form sodium dendrites. For the alkali metal battery of this application embodiment, when the first fast ion conductor is used in conjunction with the positive electrode active material and a solvent containing linear ethers and cyclic ethers, it can further facilitate the desolvation of sodium ions and the kinetics of their deposition on the negative electrode. A protective layer can be formed on the surface of the positive electrode active material layer, which can effectively improve the long-cycle stability of the sodium metal battery. At the same time, due to the reduction of internal short circuits and side reactions, the storage performance and gas generation performance of the sodium secondary battery are also improved.

[0168] In some embodiments of this application, when it is a negative electrode-free battery, the negative electrode sheet includes a negative electrode current collector and an interface modification layer disposed on at least one side of the negative electrode current collector, the interface modification layer including an adhesive and a conductive agent.

[0169] The binder and conductive agent of the negative electrode sheet have been described in detail above and will not be repeated here.

[0170] In some embodiments of this application, the thickness of the interface modification layer is 0.6μm-2μm, for example, it can be 0.6μm-1.9μm, 0.8μm-1.7μm, 1μm-1.5μm, etc.

[0171] In some embodiments of this application, the sodium metal battery includes a negative electrode-free sodium metal battery. That is, in the sodium metal battery, the negative electrode includes a negative current collector, and metallic sodium is deposited in situ on the negative current collector during charging. Experiments show that, for the above-mentioned negative electrode-free sodium metal battery, the first fast ion conductor, when used in conjunction with the positive electrode active material and solvents containing linear and cyclic ethers, has a significant effect on reducing sodium dendrites, effectively improving the cycle stability of the negative electrode-free sodium metal battery, which can cycle for more than 1000 cycles.

[0172] It's understandable that a "negative electrode-free battery" refers to a battery where no negative electrode active material is added during the battery manufacturing stage. However, a negative electrode current collector is still present. A negative electrode-free battery is simply a special type of metal battery (such as lithium metal batteries or sodium metal batteries), not a battery that truly lacks a negative electrode. In actual operation, the negative electrode still contains an active metal (such as lithium metal or sodium metal). The negative electrode in a negative electrode-free battery includes a bare negative electrode current collector (such as copper). Taking a lithium battery as an example, during battery charging, active metal ions such as Li... + The lithium metal is extracted from the positive electrode and deposited on the negative electrode current collector to form a lithium negative electrode. During subsequent battery discharge, the deposited lithium metal dissolves and is reinserted into the positive electrode.

[0173] In some embodiments of this application, the battery includes a sodium metal negative electrode sheet, wherein the negative electrode sheet includes a negative current collector and an active material layer disposed on at least a portion of the surface of the negative current collector, the active material layer comprising at least one of elemental lithium metal or a lithium metal alloy. Alternatively, the active material comprises at least one of sodium metal or a sodium metal alloy.

[0174] In some other embodiments of this application, the sodium metal alloy has the chemical formula NaR1, where R1 includes at least one of tin, zinc, aluminum, magnesium, silver, gold, gallium, indium, platinum, boron, carbon, or silicon.

[0175] In some other embodiments of this application, when a sodium metal negative electrode sheet is used, the preparation method is as follows: sodium foil or sodium metal alloy is coated onto the current collector by single-sided rolling, and then cut into negative electrode sheets.

[0176] This application does not impose any particular restrictions on the type of separator membrane; any known porous separator membrane with good chemical and mechanical stability can be selected.

[0177] In some embodiments of this application, the material of the separator may include at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, or polyvinylidene fluoride. The separator may be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer may be the same or different, without particular limitation.

[0178] The secondary batteries of this application include single-cell battery forms, battery module forms, and battery pack forms. The following description, with appropriate reference to the accompanying drawings, will illustrate the single-cell battery, battery module, and battery pack of this application.

[0179] In some embodiments of this application, the positive electrode, the negative electrode, and the separator can be fabricated into an electrode assembly by a winding process or a stacking process.

[0180] In some embodiments of this application, the battery cell may include an outer packaging. This outer packaging can be used to encapsulate the aforementioned electrode assembly and electrolyte.

[0181] In some embodiments of this application, the outer packaging of the battery cell can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the secondary battery can also be a soft pack, such as a pouch-type soft pack. The material of the soft pack can be plastic, and examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0182] It is understood that the sodium secondary battery described in this application is a single battery cell.

[0183] This application does not impose any particular limitation on the shape of the battery cell; it can be cylindrical, square, or any other arbitrary shape. For example, Figure 1 shows a square battery cell 1 as an example.

[0184] In some embodiments of this application, referring to FIG2, the outer packaging may include a housing 11 and a cover plate 13. The housing 11 may include a bottom plate and side plates connected to the bottom plate, the bottom plate and side plates forming a receiving cavity. The housing 11 has an opening communicating with the receiving cavity, and the cover plate 13 can cover the opening to close the receiving cavity. The positive electrode sheet, negative electrode sheet, and separator can be formed into an electrode assembly 12 by a winding process or a stacking process. The electrode assembly 12 is encapsulated within the receiving cavity. Electrolyte is immersed in the electrode assembly 12. The number of electrode assemblies 12 contained in the battery cell 1 can be one or more, which can be selected by those skilled in the art according to specific practical needs.

[0185] In some embodiments of this application, individual battery cells can be assembled into a battery module. The number of individual battery cells contained in a battery module can be one or more, and the specific number can be selected by those skilled in the art based on the application and capacity of the battery module.

[0186] Figure 3 shows a battery module 2 as an example. Referring to Figure 3, in battery module 2, multiple battery cells 1 can be arranged sequentially along the length of battery module 2. Of course, they can also be arranged in any other manner. Furthermore, these multiple battery cells 1 can be fixed in place using fasteners.

[0187] Optionally, the battery module 2 may also include a housing with a receiving space in which multiple battery cells 1 are received.

[0188] In some embodiments of this application, the battery modules described above can also be assembled into a battery pack. The number of battery modules contained in the battery pack can be one or more, and the specific number can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0189] Figures 4 and 5 show an example battery pack 3. Referring to Figures 4 and 5, the battery pack 3 may include a battery box and multiple battery modules 2 disposed within the battery box. The battery box includes an upper box 31 and a lower box 32, with the upper box 31 covering the lower box 32 to form a closed space for accommodating the battery modules 2. The multiple battery modules 2 can be arranged in any manner within the battery box.

[0190] In addition, this application also provides an electrical device, which includes the sodium secondary battery provided in the first aspect of this application. The battery cell, battery module, or battery pack can be used as a power source for the electrical device, or as an energy storage unit for the electrical device. The electrical device may include, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0191] As the electrical equipment, battery cells, battery modules, or battery packs can be selected according to their usage requirements.

[0192] Figure 6 shows an example of an electrical device. This device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. To meet the high power and high energy density requirements of the secondary battery for this device, a battery pack or battery module can be used.

[0193] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a single battery cell as their power source.

[0194] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0195] Example 1

[0196] 1. Preparation of positive electrode sheet

[0197] First, 10 wt% polyvinylidene fluoride binder is fully dissolved in N-methylpyrrolidone, and then 10 wt% carbon black conductive agent, 78 wt% positive electrode active material Na3V2(PO4)3 and 2 wt% NFPP are added to make a uniformly dispersed slurry, thus obtaining the slurry of the positive electrode active material layer.

[0198] 2.5 wt% polyvinylidene fluoride binder was fully dissolved in N-methylpyrrolidone, and 2.5 wt% carbon black conductive agent and 95 wt% NFPP second fast ion conductor were added to prepare a uniformly dispersed slurry (wherein, the mass percentage of residual alkali in NFPP is 0.3%, the volume average particle size Dv50 of NFPP is 2.5 μm, and the mass percentage of carbon coating material is 2%), thus obtaining the second layer slurry. The mass ratio of positive electrode active material to second fast ion conductor is 95:5.

[0199] The two slurries were uniformly coated on the surface of aluminum foil by double-layer extrusion coating (NFPP accounted for 6.65% of the total mass of the positive electrode active material and NFPP), and then transferred to a vacuum drying oven for complete drying. The resulting electrode sheet was rolled and then punched to obtain the positive electrode sheet.

[0200] The final positive electrode has an electronic conductivity of 30 μS·cm. -1 The areal density of the positive electrode active material layer is 9.5 mg / cm³.2 The thickness of the positive electrode active material layer is 52.8 μm; the areal density of the second layer is 0.5 mg / cm³. 2 The thickness of the second layer is 2.78 μm; the total compaction density of the positive electrode active material layer and the second layer is 1.8 g / cm³. 3 The compaction density of the positive electrode active material layer is 1.8 g / cm³. 3 The compaction density of the second layer is 1.8 g / cm³. 3 .

[0201] 2. Preparation of negative electrode sheet

[0202] 5g of sodium carboxymethyl cellulose (CMC) was dissolved in 1000mL of water, and then 5g of single-walled carbon nanotubes were added. After ultrasonic dispersion, a slurry was prepared. The slurry was then coated onto the surface of a copper foil and completely dried in a vacuum drying oven. Afterward, it was slit and die-cut to prepare a negative electrode sheet without a negative electrode structure. The coating weight of the negative electrode was 0.1mg / cm³. 2 The coating thickness is 1 μm.

[0203] 3. Preparation of electrolyte

[0204] In an argon atmosphere glove box (H2O < 0.1 ppm, O2 < 0.1 ppm), a certain amount of sodium hexafluorophosphate is dissolved in a mixed solvent of ethylene glycol dimethyl ether (DME) and 1,3-dioxane (where the volume ratio of DME to 1,3-dioxane is 70:30), stirred evenly, and the concentration of sodium hexafluorophosphate is controlled at 1 mol / L to form the final electrolyte.

[0205] 4. Separating membrane

[0206] Polypropylene film is used as the separator.

[0207] 5. Preparation of secondary batteries

[0208] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator acting as a barrier between the positive and negative electrodes. Tabs are welded onto the bare cell, which is then placed in an aluminum casing and baked at 80°C to remove moisture. The electrolyte is then injected and the casing is sealed, resulting in a non-charged battery. This non-charged battery then undergoes a series of processes including settling, hot and cold pressing, formation, shaping, and capacity testing to obtain a sodium-free negative electrode secondary battery.

[0209] The preparation methods of sodium secondary batteries in Examples 2-17 and Comparative Examples 1-4 are the same as those in Example 1, except that the electrolyte preparation process is different, as shown in Table 1.

[0210] In Table 1, the percentages of the solvent components represent the percentage of a particular component by volume of the total solvent. For example, the solvent in Example 1 includes 70% DME and 30% 1,3-dioxane by volume. Comparative Example 1 does not include a first fast ion conductor or a second fast ion conductor. The second layer includes 50 wt% polyvinylidene fluoride binder and 50 wt% carbon black conductive agent. In Example 17 and Comparative Examples 1-2, the areal density of the positive electrode active material layer is 8 mg / cm³. 2 The thickness of the positive electrode active material layer is 28.6 μm; the areal density of the second layer is 2 mg / cm³. 2 The second layer has a thickness of 10 μm; the total compaction density of the positive electrode active material layer and the second layer is 2.59 g / cm³. 3 The compaction density of the positive electrode active material layer is 2.8 g / cm³; the compaction density of the second layer is 2 g / cm³. 3 .

[0211] Table 1

[0212] The storage capacity retention, cycle performance and storage gas production volume of the batteries of Examples 1-17 and Comparative Examples 1-4 were characterized, and the characterization results are shown in Table 2.

[0213] 1. Storage capacity retention test

[0214] A sodium-ion battery was charged at 25°C with a constant current of 0.2C to 3.65V, then charged at a constant voltage of 3.65V until the current dropped to 0.05C, and then discharged at a constant current of 0.2C to 1.5V, yielding the initial discharge capacity (Cd1). This battery was then charged again with a constant current of 0.2C to 3.65V, and then charged at a constant voltage of 3.65V until the current dropped to 0.05C. The battery was then stored in a 60°C constant temperature chamber for 30 days. After removal, the battery was charged at 25°C with a constant current of 0.2C to 3.65V, then charged at a constant voltage of 3.65V until the current dropped to 0.05C, and then discharged at a constant current of 0.2C to 1.5V, yielding the post-storage discharge capacity (Cd2). The capacity retention rate of the sodium-ion battery was calculated using the following formula:

[0215] Storage capacity retention rate = Cd2 / Cd1 × 100%.

[0216] 2. Cyclic performance test

[0217] The cycle performance test process is as follows: At 25°C, the prepared battery was left to stand for 30 minutes, then discharged at a constant current of 0.33C to 3.65V, followed by constant voltage charging at 3.65V until the current drops to 0.05C. After standing for 1 hour, it was discharged at a constant current of 0.33C to 1.5V to obtain the initial capacity (C0). After standing for 1 hour, it was charged again at a constant current of 0.33C to 3.65V, followed by constant voltage charging at 3.65V until the current drops to 0.05C. After standing for 1 hour, it was discharged at a constant current of 0.33C to 1.5V to obtain the process capacity (C1). The above steps were repeated for the same battery, and the process capacity (Cn) of the battery after the 100th cycle was recorded. The capacity retention rate after 100 cycles = Cn / C1 × 100%. The test process for the comparative example and other embodiments was the same.

[0218] 3. Gas production volume test during storage

[0219] Before capacity testing, the cell volume (V1) was measured at 25°C using the water displacement method. The sodium-ion battery was charged at 25°C with a constant current of 0.2C to 3.65V, then charged at a constant voltage of 3.65V until the current dropped to 0.05C, and then discharged at a constant current of 0.2C to 1.5V, yielding the discharge capacity before storage (Cd1). This battery was then charged again with a constant current of 0.2C to 3.65V, followed by a constant voltage charge at 3.65V until the current dropped to 0.05C. The battery was then stored in a 60°C constant temperature chamber for 30 days. After removal, the cell volume (V2) was measured at 25°C, and the gas production of the sodium-ion battery was calculated using the following formula:

[0220] Gas production = [volume of cell after storage (V2) - volume of cell before storage (V1)] / cell capacity Cd1, the results are shown in Table 2.

[0221] Table 2

[0222] As can be seen from Table 2, in the sodium secondary batteries of Examples 1-17 of this application, the positive electrode active material layer includes a positive electrode active material layer and a first fast ion conductor. Combined with linear ether and cyclic ether in the electrolyte solvent, the sodium secondary battery exhibits excellent cycle performance, low gas production, and good high-temperature storage performance. Comparative Examples 1 and 3 did not use a combination of positive electrode active material and first fast ion conductor, and Comparative Examples 2 and 4 did not use linear ether and cyclic ether in the electrolyte. The cycle performance, storage performance, and gas production of the batteries were significantly deteriorated. It is evident that the sodium secondary batteries of the embodiments of this application can reduce the internal shortness caused by the growth of sodium dendrites in the sodium secondary battery, improve the cycle performance and storage performance of the sodium secondary battery, and reduce gas production.

[0223] The differences between the sodium secondary batteries in Examples 18 and 19 and those in Example 17 are shown in Table 3. The chemical formula of NMVP is Na4MnV(PO4)3, and the chemical formula of NMTP is Na3MnTi(PO4)3. The performance testing methods for the sodium secondary batteries are the same as described above.

[0224] Table 3

[0225] As can be seen from Table 3, in Examples 17-19, the sodium secondary batteries exhibited excellent cycle performance, low gas production, and good high-temperature storage performance when using different first fast ion conductors.

[0226] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A sodium secondary battery, wherein, The sodium secondary battery includes a positive electrode, an electrolyte, and a negative electrode, wherein... The positive electrode sheet includes a positive current collector and a positive active material layer disposed on at least one side of the positive current collector. The positive active material layer includes a positive active material and a first fast ion conductor. The first fast ion conductor includes one or more of sodium pyrophosphate, sodium manganese vanadium phosphate, and sodium titanium manganese phosphate. The electrolyte includes a solvent, which includes linear ethers and cyclic ethers.

2. The sodium secondary battery according to claim 1, wherein, The sodium secondary battery satisfies one or more of the following conditions: The sodium pyrophosphate salt includes Na 4+a M 3+b (PO4) 2+c (P2O7) 1+d Wherein, M includes at least one of Fe, Co, Mn or Ni, and -0.1≤a≤0.1, -0.1≤b≤0.1, -0.1≤c≤0.1, -0.1≤d≤0.1; The sodium manganese vanadium phosphate includes Na 3+f Mn x V 2-x (PO4) 3+y1 Where -0.1≤f≤0.1, 0≤x≤1, -0.1≤y1≤0.1; The sodium titanium manganese phosphate includes Na 3+x1 MnTi 1-x1 V x1 (PO4) 3+e , where 0≤x1≤1, -0.1≤e≤0.

1.

3. The sodium secondary battery according to claim 1 or 2, wherein, The positive electrode further includes a second layer disposed on the side of the positive active material layer away from the positive current collector. The second layer includes a second fast ion conductor, which includes one or more of sodium pyrophosphate, sodium manganese vanadium phosphate, and sodium titanium manganese phosphate.

4. The sodium secondary battery according to any one of claims 1-3, wherein, In the first fast ion conductor, the mass percentage of residual alkali is 0.05%-2.5%, and can be selected as 0.05%-0.5%.

5. The sodium secondary battery according to any one of claims 1-4, wherein, The volume ratio of the linear ether to the cyclic ether is (60:40)-(90:10).

6. The sodium secondary battery according to any one of claims 1-5, wherein, At least a portion of the surface of the first fast ion conductor is covered with a carbon-coated material, and the mass percentage of the carbon-coated material is 1.5%-2.5% based on the total mass of the first fast ion conductor.

7. The sodium secondary battery according to claim 3, wherein, In the second fast ion conductor, the residual alkali content is 0.05%-2.5% by mass, optionally 0.05%-0.5%; and / or, At least a portion of the surface of the second fast ion conductor is covered with a carbon-coated material, and the mass percentage of the carbon-coated material is 1.5%-2.5% based on the total mass of the second fast ion conductor.

8. The sodium secondary battery according to claim 3, wherein, The mass ratio of the positive electrode active material to the second fast ion conductor is (80:20)-(95:5).

9. The sodium secondary battery according to claims 1-8, wherein, Based on the total mass of the positive electrode active material layer, the mass percentage of the first fast ion conductor is 1%-3%.

10. The sodium secondary battery according to claim 3 or 8, wherein, Based on the total mass of the positive electrode active material, the first fast ion conductor, and the second fast ion conductor, the sum of the masses of the first fast ion conductor and the second fast ion conductor accounts for 5%-20%; and / or, Based on the total mass of the second layer, the mass percentage of the second fast ion conductor is 90%-95%; and / or, The volume average particle size Dv50 of the second fast ion conductor is 2.5 μm-3.5 μm.

11. The sodium secondary battery according to any one of claims 1-10, wherein, The electronic conductivity of the positive electrode is greater than or equal to 25 μS·cm. -1 .

12. The sodium secondary battery according to any one of claims 1-11, wherein, The sodium secondary battery satisfies one or more of the following conditions: The electrolyte has an ionic conductivity greater than or equal to 9 mS·cm. -1 ; The linear ether includes at least one of ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, polyethylene glycol dimethyl ether, ethylene glycol diethyl ether, ethylene glycol dibutyl ether, diethylene glycol dibutyl ether, or diethylene glycol diethyl ether; The cyclic ether includes at least one of tetrahydrofuran, methyltetrahydrofuran, or 1,3-dioxane; The volume average particle size Dv50 of the first fast ion conductor is 2.5 μm-3.5 μm.

13. The sodium secondary battery according to any one of claims 1-12, wherein, The positive electrode active material includes one or more of sodium transition metal oxides and phosphate compounds; The sodium transition metal oxide includes Na. a1 M1 b1 O c1 Wherein, M1 includes one or more of Ti, V, Cr, Mn, Fe, Co, Ni, or Cu, 0.67≤a1≤1.2, 0.9≤b1≤1.1, 0.9≤c1≤2.1; and / or, The phosphate compound includes Na. x3 R y3 P m O n Wherein, 2.5≤x³≤4.5, 1.5≤y³≤3.5, 2.5<m<4.5, 11.5≤n≤15.5, and R includes one or more of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Cr, Nb, Mo, In, Ga, Sn, Hf, Ta, W, or Pb; and / or, Based on the total mass of the positive electrode active material layer, the mass ratio of the positive electrode active material is 77%-80%.

14. The sodium secondary battery according to any one of claims 3, 8, and 10, wherein, The positive electrode active material includes a sodium transition metal oxide, which includes Na. 1+x2 M1O 2+y2 Wherein, M1 includes one or more of Ti, V, Cr, Mn, Fe, Co, Ni or Cu, -0.1≤x2≤0.1, -0.1≤y2≤0.1, and the sodium secondary battery satisfies one or more of the following conditions: The areal density of the positive electrode active material layer is 8 mg / cm³. 2 -9.5mg / cm 2 ; The thickness of the positive electrode active material layer is 25μm-33.9μm; The areal density of the second layer is 0.5 mg / cm³. 2 -2mg / cm 2 ; The thickness of the second layer is 2.5μm-11.7μm; The total compaction density of the positive electrode active material layer and the second layer is 2.58 g / cm³. 3 -2.75g / cm 3 ; The compaction density of the positive electrode active material layer is 2.8 g / cm³. 3 -3.2g / cm 3 ; The compacted density of the second layer is 1.7 g / cm³. 3 -2g / cm 3 .

15. The sodium secondary battery according to any one of claims 3, 8, and 10, wherein, The positive electrode active material includes phosphate compounds, which include Na. x3 R y3 P m O n Wherein, 2.5≤x³≤4.5, 1.5≤y³≤3.5, 2.5<m<4.5, 11.5≤n≤15.5, and R includes one or more of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Cr, Nb, Mo, In, Ga, Sn, Hf, Ta, W, or Pb, and the sodium secondary battery satisfies one or more of the following conditions: The areal density of the positive electrode active material layer is 8 mg / cm³. 2 -9.5mg / cm 2 ; The thickness of the positive electrode active material layer is 40μm-55.9μm; The areal density of the second layer is 0.5 mg / cm³. 2 -2mg / cm 2 ; The thickness of the second layer is 2.5μm-11.7μm; The total compaction density of the positive electrode active material layer and the second layer is 1.7 g / cm³. 3 -2g / cm 3 ; The compaction density of the positive electrode active material layer is 1.7 g / cm³. 3 -2g / cm 3 ; The compacted density of the second layer is 1.7 g / cm³. 3 -2g / cm 3 .

16. The sodium secondary battery according to any one of claims 1-15, wherein, The sodium secondary battery includes a sodium metal battery.

17. The sodium secondary battery according to any one of claims 1-16, wherein, The negative electrode sheet includes a negative current collector and an interface modification layer disposed on at least one side of the negative current collector, the interface modification layer including an adhesive and a conductive agent.

18. An electrical appliance, wherein, The sodium secondary battery includes any one of claims 1-17.

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